Real-time correction of anatomical maps

ABSTRACT

A system includes an input device and a processor. The processor is configured to add, to a point cloud representing an anatomical volume, multiple points corresponding to respective locations of a probe within the anatomical volume, to remove a subset of the points from the point cloud in response to an input received via the input device, subsequently to adding the points, and to add, to the point cloud, other points corresponding to respective subsequent locations of the probe within the anatomical volume, subsequently to removing the subset. Other embodiments are also described.

FIELD OF THE INVENTION

The present invention relates to the field of anatomical mapping.

BACKGROUND

United States Patent Application Publication No. 2009/0148012 to Altmannet al. describes a method of medical imaging, including creating ananatomical map of an inner wall of a cavity in a body of a subject byinserting a probe into the body and collecting data using the probe. Athree-dimensional (3-D) contour is delineated in a 3-D image of thecavity based on the map.

SUMMARY OF THE INVENTION

There is provided, in accordance with some embodiments of the presentinvention, a system including a display and a processor. The processoris configured to compute a point P′ on a virtual surface of a pointcloud representing an anatomical volume, by projecting another point P,which corresponds to a location on an anatomical surface of theanatomical volume, onto the virtual surface. The processor is furtherconfigured to define a virtual sphere centered on a virtual line joiningP to P′ such that P lies on a spherical surface of the virtual sphere.The processor is further configured to perform an operation selectedfrom the group of operations consisting of: expanding the point cloudthroughout the virtual sphere, and excluding the virtual sphere from thepoint cloud. The processor is further configured to regenerate thevirtual surface, subsequently to performing the operation, such that, byvirtue of having performed the operation, P lies on the virtual surface,and to display the regenerated virtual surface on the display.

In some embodiments, the processor is configured to regenerate thevirtual surface by applying a ball-pivoting algorithm (BPA) with a ballradius r to the point cloud, and the processor is configured to definethe sphere such that a radius R of the sphere is at least 2r.

In some embodiments, R=2r.

In some embodiments, the processor is further configured to identify Pin response to an input from a user.

In some embodiments, the input indicates that an intrabody probe is atthe location.

In some embodiments, the input includes a selection of P from a displayof the point cloud.

In some embodiments, the anatomical volume includes at least part of achamber of a heart.

There is further provided, in accordance with some embodiments of thepresent invention, a method including, using a processor, computing apoint P′ on a virtual surface of a point cloud representing ananatomical volume, by projecting another point P, which corresponds to alocation on an anatomical surface of the anatomical volume, onto thevirtual surface. The method further includes defining a virtual spherecentered on a virtual line joining P to P′ such that P lies on aspherical surface of the virtual sphere. The method further includesperforming an operation selected from the group of operations consistingof: expanding the point cloud throughout the virtual sphere, andexcluding the virtual sphere from the point cloud. The method furtherincludes, subsequently to performing the operation, regenerating thevirtual surface such that, by virtue of having performed the operation,P lies on the virtual surface.

There is further provided, in accordance with some embodiments of thepresent invention, a computer software product including a tangiblenon-transitory computer-readable medium in which program instructionsare stored. The instructions, when read by a processor, cause theprocessor to compute a point P′ on a virtual surface of a point cloudrepresenting an anatomical volume, by projecting another point P, whichcorresponds to a location on an anatomical surface of the anatomicalvolume, onto the virtual surface. The instructions further cause theprocessor to define a virtual sphere centered on a virtual line joiningP to P′ such that P lies on a spherical surface of the virtual sphere.The instructions further cause the processor to perform an operationselected from the group of operations consisting of: expanding the pointcloud throughout the virtual sphere, and excluding the virtual spherefrom the point cloud. The instructions further cause the processor toregenerate the virtual surface, subsequently to performing theoperation, such that, by virtue of having performed the operation, Plies on the virtual surface.

There is further provided, in accordance with some embodiments of thepresent invention, a system including an input device and a processor.The processor is configured to add, to a point cloud representing ananatomical volume, multiple points corresponding to respective locationsof a probe within the anatomical volume. The processor is furtherconfigured to remove a subset of the points from the point cloud,subsequently to adding the points, in response to an input received viathe input device. The processor is further configured to add to thepoint cloud, subsequently to removing the subset, other pointscorresponding to respective subsequent locations of the probe within theanatomical volume.

In some embodiments,

the input indicates that the probe is to be used in a point-removalmode, and

the processor is configured to remove the subset by, for each point inthe subset, removing the point in response to ascertaining that theprobe is at a location to which the point corresponds.

In some embodiments,

the input is a first input, and

the processor is configured to add the other points to the point cloudin response to a second input indicating that the probe is to be used ina point-addition mode.

In some embodiments, the processor is configured to remove the point inresponse to ascertaining that the probe is pushing against a surface ofthe anatomical volume.

In some embodiments, the input indicates a period of time, and thesubset includes those of the points that were added to the point cloudduring the period of time.

In some embodiments, the period of time begins T units of time before atime at which the input is submitted, and the input indicates the periodof time by indicating T.

In some embodiments, the input indicates an integer N, and the subsetincludes N of the points that were most recently added to the pointcloud.

In some embodiments, the anatomical volume includes at least part of achamber of a heart.

There is further provided, in accordance with some embodiments of thepresent invention, a method including, using a processor, adding, to apoint cloud representing an anatomical volume, multiple pointscorresponding to respective locations of a probe within the anatomicalvolume. The method further includes, subsequently to adding the points,removing a subset of the points from the point cloud in response to aninput from a user. The method further includes, subsequently to removingthe subset, adding, to the point cloud, other points corresponding torespective subsequent locations of the probe within the anatomicalvolume.

There is further provided, in accordance with some embodiments of thepresent invention, a computer software product including a tangiblenon-transitory computer-readable medium in which program instructionsare stored. The instructions, when read by a processor, cause theprocessor to add, to a point cloud representing an anatomical volume,multiple points corresponding to respective locations of a probe withinthe anatomical volume. The instructions further cause the processor toremove a subset of the points from the point cloud in response to aninput from a user, subsequently to adding the points. The instructionsfurther cause the processor to add to the point cloud, subsequently toremoving the subset, other points corresponding to respective subsequentlocations of the probe within the anatomical volume.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood from the followingdetailed description of exemplary embodiments thereof, taken togetherwith the drawings, in which:

FIG. 1 is a schematic illustration of an anatomical mapping system, inaccordance with some exemplary embodiments of the present invention;

FIG. 2 is a schematic illustration of a technique for generating ananatomical map, in accordance with some exemplary embodiments of thepresent invention;

FIG. 3 is a flow diagram for an algorithm for generating an anatomicalmap, in accordance with some exemplary embodiments of the presentinvention;

FIGS. 4A-B are schematic illustrations of a technique for reconstructinga model, in accordance with some exemplary embodiments of the presentinvention; and

FIG. 5 is a flow diagram for an algorithm for reconstructing a model, inaccordance with some exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

In an anatomical mapping of a chamber of a heart, a physician moves aprobe through the chamber while a tracking system tracks the location ofthe probe. Based on the tracking, a model of the chamber is constructed.The model includes a point cloud, which includes multiple pointscorresponding to the locations of the probe (typically, of the distalend of the probe) within the chamber, and a surface surrounding thepoint cloud, which represents the tissue surrounding the chamber. Thesurface may be associated with electrical properties of the tissue,which may be derived from electrogram signals acquired by the probe atvarious locations on the tissue.

A challenge, when performing such a mapping, is that spurious points maybe added to the model, e.g., due to respiratory movement or excess forceapplied to the tissue by the probe. Conventionally, these points areremoved after the mapping by modifying the surface of the model (e.g.,using spline deformation and/or surface interpolation) such that thesurface passes through points that are known to correspond to locationson the tissue. (These points, referred to herein as “anchor points,” maycorrespond to those locations at which electrogram signals wereacquired, since the physician is generally careful, when acquiring theelectrogram signals, not to push against the tissue with excess force.)However, this deformation may compromise the accuracy of the model.

To address this challenge, embodiments of the present invention providevarious techniques for removing spurious points from the model.Following the removal of the spurious points, the surface may beregenerated. Thus, it may not be necessary to deform the surface.

Per one such technique, a point-removal mode for operation of the probeis provided. While the probe is operated in this mode, movement of theprobe to a particular location causes the points corresponding to thislocation to be removed from the model. The physician may thus correctthe model in real-time, by switching from the usual point-addition modeto the point-removal model, and then moving the probe to any locationsthat correspond to spurious points.

Per another such technique, for each anchor point, a virtual spherewhose surface passes through the anchor point is defined (as describedin detail below with reference to FIG. 4A), and any points within thesphere are then removed from the point cloud. Subsequently, the modelsurface—or at least the portion of the model surface near the anchorpoint—is regenerated, such that the model surface passes through theanchor point. Advantageously, this technique may be performed followingthe acquisition of each new anchor point, during or following theconstruction of the point cloud.

For cases in which an anchor point lies outside the point cloud, avariation of the latter technique may be performed. Per this variation,following the definition of the virtual sphere, the sphere is filledwith additional points so as to augment the point cloud.

System Description

Reference is initially made to FIG. 1 , which is a schematicillustration of an anatomical mapping system 20, in accordance with someembodiments of the present invention.

System 20 comprises an intrabody probe 26 proximally connected to aconsole 32. System 20 further comprises a processor 34, which istypically contained in console 32.

System 20 is used to generate an anatomical map 38 of a volume of theheart 24 of a subject 22, such as at least part of a chamber of heart24. Map 38 includes a point cloud, which represents the mapped volume byvirtue of including a plurality of points corresponding to respectivelocations within the mapped volume. Map 38 further includes a virtualsurface 46 around the periphery of the point cloud. While the map isbeing generated, and/or subsequently to the generation of the map, theprocessor may display the map on a display 36.

More specifically, as a physician 30 moves probe 26 within the volume,processor 34 continually ascertains the sub-volume occupied by thedistal end of the probe. The processor further constructs the pointcloud by adding to the point cloud, for each ascertained sub-volume 27,one or more points—typically, multiple points—corresponding tosub-volume 27. (Each added point thus corresponds to a respectivelocation of the probe—in particular, of any portion of the distal end ofthe probe—within the mapped volume.) Subsequently to constructing thepoint cloud, and/or intermittently while the point cloud is constructed,the processor generates virtual surface 46. (It is noted that for easeof description, sub-volume 27 may be referred to herein as the “locationof the probe.”)

Typically, the distal end of probe 26 comprises at least one sensingelectrode 28. As the probe is moved within the heart, electrode 28acquires electrogram signals from tissue of the heart. Processor 34analyzes these signals and, in response thereto, annotates map 38 (e.g.,by coloring the map per a sliding color scale) to indicate electricalproperties of the tissue. In such embodiments, map 38 may be referred toas an “electroanatomical map” or an “electrophysiological map.”

In some embodiments, to facilitate tracking the probe, one or moreelectromagnetic coils are coupled to the distal end of the probe, and amagnetic field is generated in the vicinity of subject 22. As the probeis moved within the heart, the magnetic field induces alocation-dependent signal in the coils. Based on this signal, theprocessor ascertains the location of each of the coils, and hence,identifies sub-volume 27. Such magnetic-based tracking is disclosed, forexample, in U.S. Pat. Nos. 5,391,199, 5,443,489, and 6,788,967 toBen-Haim, U.S. Pat. No. 6,690,963 to Ben-Haim et al., U.S. Pat. No.5,558,091 to Acker et al., and U.S. Pat. No. 6,177,792 to Govari, whoserespective disclosures are incorporated herein by reference.

Alternatively or additionally, one or more electrodes coupled to thedistal end of the probe may pass electric currents to a plurality ofelectrode patches coupled to the subject's body at different respectivelocations. (Typically, these electrodes do not include electrode 28.)Based on the electric currents and the body-impedance measurementsderived therefrom, the processor may ascertain the location of each ofthe electrodes, and hence, identify sub-volume 27. A hybrid techniquecombining such impedance-based tracking with magnetic-based tracking isdescribed in U.S. Pat. No. 8,456,182 to Bar-Tal et al., whose disclosureis incorporated herein by reference.

Alternatively or additionally, the processor may track the probe, andhence construct the point cloud, using any other suitable trackingtechnique.

In general, processor 34 may be embodied as a single processor, or as acooperatively networked or clustered set of processors. In someembodiments, the functionality of processor 34, as described herein, isimplemented solely in hardware, e.g., using one or moreApplication-Specific Integrated Circuits (ASICs) or Field-ProgrammableGate Arrays (FPGAs). In other embodiments, the functionality ofprocessor 34 is implemented at least partly in software. For example, insome embodiments, processor 34 is embodied as a programmed digitalcomputing device comprising at least a central processing unit (CPU) andrandom-access memory (RAM). Program code, including software programs,and/or data are loaded into the RAM for execution and processing by theCPU. The program code and/or data may be downloaded to the processor inelectronic form, over a network, for example. Alternatively oradditionally, the program code and/or data may be provided and/or storedon non-transitory tangible media, such as magnetic, optical, orelectronic memory. Such program code and/or data, when provided to theprocessor, produce a machine or special-purpose computer, configured toperform the tasks described herein.

Generating the Anatomical Map

Reference is now made to FIG. 2 , which is a schematic illustration of atechnique for generating map 38, in accordance with some embodiments ofthe present invention.

FIG. 2 shows a portion of a point cloud 42, which, as described abovewith reference to FIG. 1 , represents the mapped volume of the heart byvirtue of including a plurality of points 44 corresponding to respectivelocations of probe 26 within the heart. FIG. 2 further shows virtualsurface 46, which represents the anatomical surface 40 of the mappedvolume (i.e., the tissue surrounding the mapped volume).

Typically, the processor generates virtual surface 46 by applying theball-pivoting algorithm (BPA), described in Bernardini, Fausto, et al.,“The ball-pivoting algorithm for surface reconstruction,” IEEEtransactions on visualization and computer graphics 5.4 (1999): 349-359,which is incorporated herein by reference, to the point cloud. Thisalgorithm computes surface 46 as a triangulated mesh that approximatelyinterpolates the points at the periphery of the point cloud, by (i)finding a seed triangle whose vertices are points at the periphery ofthe point cloud, and then (ii) expanding the triangulation by pivoting avirtual ball around the edges of the existing triangles until no openedges remain. Alternatively, the processor may use any other suitablesurface-generating algorithm.

The leftmost portion of FIG. 2 illustrates a scenario in which probe 26pushes surface 40 outward from the natural position 48 of the surface,so as to create a bulge 50 in the surface. Consequently, as shown in themiddle portion of FIG. 2 , spurious points, which correspond to one ormore locations of the probe within bulge 50, are added to the pointcloud, such that virtual surface 46 acquires a spurious bulge 52.

To address this problem, embodiments of the present invention providevarious techniques for removing a subset of points 44 from the pointcloud and then regenerating virtual surface 46, as shown in therightmost portion of FIG. 2 . Subsequently to the removal of the subset,other points may be added to the point cloud. In this manner, spuriouspoints may be removed during the mapping procedure, thus reducing therequired post-processing of the map.

Typically, the removal of the subset of points is initiated by a user ofthe system. For example, in response to noticing bulge 52 on display 36(FIG. 1 ), a user, such as physician 30, may submit an input indicatingthat some of the points should be removed from the point cloud.Subsequently, in response to the input, the processor may remove thesubset of points.

In some embodiments, the probe may be used in two different modes: apoint-addition mode, and a point-removal mode. While the probe is in thepoint-addition mode, the processor adds respective points correspondingto each new location of the probe. On the other hand, while the probe isin the point-removal mode, the processor removes any pointscorresponding to the location of the probe. (In addition, the probe mayhave a third, inactive mode, in which points are neither added norremoved.)

In such embodiments, the aforementioned input from the user indicatesthat the probe is to be used in the point-removal mode, and theprocessor removes each point in the subset in response to ascertainingthat the probe is at a location to which the point corresponds. Forexample, subsequently to adding a first group of points to the pointcloud, the physician may submit an input indicating that the probe is tobe used in the point-removal mode. Subsequently, the physician may passthe probe through a previously-mapped portion of the subject's anatomy.In response to the input and to ascertaining the locations of the probewithin this portion, the processor may remove the points correspondingto these locations. Next, after removing the probe from this portion,the physician may submit another input indicating that the probe is tobe used, again, in the point-addition mode. In response to this input,the processor may again add points to the point cloud.

To reduce the number of legitimate points removed from the point cloud,the processor may, before removing a point, check whether the probe ispushing against surface 40, e.g., by processing a signal from a pressuresensor at the distal end of the probe. In response to ascertaining thatthe probe is pushing against the surface, the processor may remove thepoint; otherwise, the processor may refrain from removing the point.

Alternatively or additionally to operating the probe in a point-removalmode, other techniques may be used to remove points from the pointcloud. For example, the point-removal-initiating input from the user mayindicate a period of time, and the removed subset of points may includethose of the points that were added to the point cloud during the periodof time. As a specific example, the period of time may begin T units oftime (e.g., seconds) before the time at which the input is submitted,and the input may indicate the period of time by indicating T.Alternatively, the input may indicate an integer N, and the subset mayinclude the N points that were most recently added to the point cloud.

In general, the user may use any suitable input device to submit theaforementioned inputs. For example, the user may press one or morebuttons or keys on console 32 (FIG. 1 ) or on a control handle for theprobe. Alternatively or additionally, the display may comprise a touchscreen, and the user may submit at least some of the inputs via thetouch screen. Alternatively or additionally, the user may submit atleast some of the inputs using a keyboard and/or mouse. As yet anotheroption, the user may use a foot pedal, e.g., to toggle between the modesin which the probe is operated.

Reference is now made to FIG. 3 , which is a flow diagram for analgorithm 54 for generating map 38 (FIG. 1 ), in accordance with someembodiments of the present invention.

Per algorithm 54, the processor repeatedly ascertains the location ofthe probe at a location-ascertaining step 56. Subsequently toascertaining the location of the probe, the processor checks, at a firstchecking step 58, whether the probe is in the point-addition mode. Ifyes, the processor adds, to the point cloud, one or more pointscorresponding to the location, at a point-adding step 60. Otherwise, theprocessor checks, at a second checking step 62, whether the probe is inthe point-removal mode. If yes, the processor removes one or more pointscorresponding to the location, at a point-removing step 64. Subsequentlyto adding or removing points, or if the probe is neither in thepoint-addition mode nor in the point-removal mode (but rather, is in theinactive mode), the processor returns to location-ascertaining step 56.

In parallel to algorithm 54, the processor may execute another algorithmfor receiving input from the user and toggling between the modes inresponse thereto. Additionally, in parallel to algorithm 54, theprocessor may execute another algorithm for intermittently regeneratingvirtual surface 46 (FIG. 2 ). For example, the processor mayintermittently regenerate part of the virtual surface at each peripheralportion of the point cloud at which points have been added or removed.

Using the Anchor Points

In some embodiments, the processor identifies one or more anchor pointsknown to correspond to locations on the anatomical surface. For eachanchor point, the processor may remove a portion of the point cloudlying outside the anchor point, and then regenerate the virtual surfacesuch that the virtual surface passes through the anchor point. (Thistechnique may be performed alternatively or additionally to thepoint-removal techniques described above.) Alternatively, the processormay expand the point cloud such that, following the regeneration of thevirtual surface, the virtual surface passes through the anchor point.

Typically, the processor identifies each anchor point in response to aninput from a user, such as physician 30 (FIG. 1 ). For example, asdescribed above in the Overview, during the mapping, electrogram signalsmay be acquired at various locations on the anatomical surface. At eachof these locations, the physician may place the probe in contact withthe tissue, typically without pushing the tissue outward, and thensubmit an input to the processor indicating that the probe is located onthe tissue. (In general, the user may use any suitable input device tosubmit this input, as described above with reference to FIG. 2 .) Inresponse to such an input, the processor may identify, as an anchorpoint, a point corresponding to the location of the probe. (It is notedthat the anchor point may already belong to the point cloud, by virtueof the probe having previously been at the current location.)Alternatively or additionally, while the point cloud is displayed ondisplay 36 (FIG. 1 ), the user may select, as the anchor point, one ofthe points belonging to the point cloud, e.g., using a touch screen ormouse to perform this selection.

For further details, reference is now made to FIGS. 4A-B, which areschematic illustrations of a technique for reconstructing a model, inaccordance with some embodiments of the present invention.

The leftmost portion of FIG. 4A illustrates a scenario in which thepoint cloud, and hence virtual surface 46, have been erroneouslyexpanded outward. (The shaded portions of FIGS. 4A-B correspond to thepoint cloud.) In this scenario, the processor, as shown in the middleportion of FIG. 4A, removes points from the point cloud such that ananchor point P, which corresponds to a location on the anatomicalsurface, is at the periphery of the point cloud. Subsequently, as shownin the rightmost portion of FIG. 4A, the processor regenerates virtualsurface 46 such that, by virtue of P being at the periphery of the pointcloud, P lies on the virtual surface.

In particular, subsequently to identifying point P, the processorcomputes another point P′ on the virtual surface, by projecting P ontothe virtual surface. In other words, the processor computes P′ as thepoint on the virtual surface closest to P. (P′ may be an original pointbelonging to the point cloud, though typically P′ is a new point definedby the processor.) Subsequently, the processor defines a virtual sphere43 centered on (i.e., having a center 45 that lies on) a virtual line 47joining P to P′ such that P lies on the (spherical) surface of virtualsphere 43. Next, the processor excludes the virtual sphere from thepoint cloud, i.e., the processor removes, from the point cloud, eachpoint lying within the sphere. Finally, the processor regenerates thevirtual surface such that, by virtue of having excluded the virtualsphere from the point cloud, P lies on the virtual surface.

Typically, the virtual surface is regenerated using the BPA. In suchembodiments, the radius R of the sphere is at least 2r, r being theradius of the virtual ball 49 used for the BPA, such that ball 49 may bepivoted within the region from which the points were removed. Forexample, R may equal 2r.

As illustrated in FIG. 4B, the above-described technique may also beused to advance the point cloud and virtual surface outward, in theevent that a peripheral portion of the anatomical volume was missedduring the mapping. In other words, the processor may identify P′ andthen define the virtual sphere, as described above and shown in theleftmost portion of FIG. 4B. Subsequently, the processor may expand thepoint cloud throughout the virtual sphere, i.e., fill the portion of thesphere lying outside the point cloud with new points, as shown in themiddle portion of FIG. 4B. (The new points may have any suitableinter-point spacing.) Finally, as shown in the rightmost portion of FIG.4B, the processor may regenerate the virtual surface as described above,such that, by virtue of having expanded the point cloud throughout thevirtual sphere, P lies on the virtual surface.

Reference is now made to FIG. 5 , which is a flow diagram for analgorithm 66 for reconstructing a model, in accordance with someembodiments of the present invention.

Algorithm 66 begins with a point-identifying step 68, at which theprocessor identifies the anchor point P. Subsequently, at a projectingstep 70, the processor computes point P′ by projecting P onto thevirtual surface. Next, at a virtual-sphere-defining step 72, theprocessor defines the virtual sphere. Following the definition of thevirtual sphere, the processor, at a point-cloud-modifying step 74,modifies the point cloud with respect to the virtual sphere (by addingpoints to, or removing points from, the virtual sphere) so as to cause Pto be at the periphery of the point cloud. Finally, at a regeneratingstep 76, the processor regenerates the virtual surface such that,subsequently, P lies on the virtual surface.

Although the above description mainly relates to the mapping of achamber of a heart, it is noted that the techniques described herein mayalso be applied to the mapping of any other anatomical volume, such asan otolaryngological volume.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of embodiments of the presentinvention includes both combinations and subcombinations of the variousfeatures described hereinabove, as well as variations and modificationsthereof that are not in the prior art, which would occur to personsskilled in the art upon reading the foregoing description. Documentsincorporated by reference in the present patent application are to beconsidered an integral part of the application except that to the extentany terms are defined in these incorporated documents in a manner thatconflicts with the definitions made explicitly or implicitly in thepresent specification, only the definitions in the present specificationshould be considered.

The invention claimed is:
 1. A system for real-time correction ofanatomical maps, the system, comprising: an input device; and aprocessor, configured to: add, to a point cloud representing ananatomical volume, multiple points corresponding to respective locationsof a probe within the anatomical volume, subsequently to adding thepoints, remove a subset of the points from the point cloud in responseto an input received via the input device, wherein the input indicates aperiod of time, and wherein the subset includes those of the points thatwere added to the point cloud during the period of time and wherein theperiod of time begins T units of time before a time at which the inputis submitted, and wherein the input indicates the period of time byindicating T, and subsequently to removing the subset, add, to the pointcloud, other points corresponding to respective subsequent locations ofthe probe within the anatomical volume.
 2. The system according to claim1, wherein the input indicates an integer N, and wherein the subsetincludes N of the points that were most recently added to the pointcloud.
 3. The system according to claim 1, wherein the anatomical volumeincludes at least part of a chamber of a heart.
 4. A method forreal-time correction of anatomical maps, the method comprising: using aprocessor, adding, to a point cloud representing an anatomical volume,multiple points corresponding to respective locations of a probe withinthe anatomical volume; subsequently to adding the points, removing asubset of the points from the point cloud in response to an input from auser, wherein the input indicates a period of time, and wherein thesubset includes those of the points that were added to the point cloudduring the period of time and wherein the period of time begins T unitsof time before a time at which the input is submitted, and wherein theinput indicates the period of time by indicating T; and subsequently toremoving the subset, adding, to the point cloud, other pointscorresponding to respective subsequent locations of the probe within theanatomical volume.
 5. The method according to claim 4, wherein the inputindicates an integer N, and wherein the subset includes N of the pointsthat were most recently added to the point cloud.
 6. The methodaccording to claim 4, wherein the anatomical volume includes at leastpart of a chamber of a heart.